1. Field of the Invention
The present invention relates to a DS-CDMA multi-user interference canceller and a DS-CDMA communication system in a DS-CDMA (Direct Sequencexe2x80x94Code Division Multiple Access) communication system.
2. Description of the Related Art
Recently, the demand for portable mobile devices has been remarkably increased, and a DS-CDMA communication system or an FH (Frequency Hopping)-CDMA communication system are expected as a system for satisfying this demand. Further, it is being promoted to use base stations and a plurality of portable mobile equipment serving as mobile stations which use the above system.
According to the DS-CDMA communication system, voice signals and signal data is modulated by the DS (Direct Sequence)-based SS (Spread Spectrum) system. This system has various advantages of multi-user performance, high resistance to interference, high secrecy performance, etc., and high-quality telephone communications and data communications can be satisfied by the base stations and the plural mobile devices. However, when a number of users communicate with a base station in the same frequency band, the communication of a user may be disturbed due to interference of another user""s communication. As compared with a frequency modulation system or a time-division multiplexing system, the DS-CDMA communication system has higher resistance to interference, however, restriction is imposed on the number of users which can communicate in the same frequency band.
In view of the foregoing problem of the DS-CDMA communication system, a multi-user serial interference canceller has been proposed, and also a method of reducing the interference in regionally-sectioned area cells and increasing the capacity for subscribers or enhancing the communication quality (QOS: Quality Of Service) has been proposed for the multi-user serial interference canceller. According to this method, before the signal of a user k (1xe2x89xa6kxe2x89xa6K: K represents the number of all users in a cell) is decoded, interference replica components of users other than the user k which have been decoded until that time are generated and subtracted from the signal of the user k to decode the signal of the user k. This processing is carried out at I times (I represents the stage number of multi-stages), thereby reducing interference effects of the other users. The interference replica components generated with the other reception signals are delayed by the processing time corresponding to the number of the other users, and the interference components of the other users are subtracted from the reception signal of the user k concerned. As the number of the other users increases, the delay time is increased more and more, resulting in occurrence of such a problem that the capacity of a memory for holding the interference replica components which have been detected until the delay time is increased and the processing time is lengthened because the processing must wait during the delay time.
In order to resolve this problem, a multi-user serial interference canceller has been proposed. The principle of the multi-user serial interference canceller is described in xe2x80x9cSEQUENTIAL CHANNEL ESTIMATION TYPE SERIAL CANCELLER USING PILOT SYMBOL IN DS-CDMAxe2x80x9d of the technical reports of ELECTRONIC INFORMATION COMMUNICATION ASSOCIATE (RCS95-50), and an example of a fabricating method thereof is disclosed in xe2x80x9cDS-CDMA MULTI-USER SERIAL INTERFERENCE CANCELLERxe2x80x9d of Japanese Laid-open Patent Application No. Hei-09-270736.
FIG. 4 shows the method disclosed in Japanese Laid-open Patent Application No. Hei-09-270736. In FIG. 4, reference numeral 31 represents a reception signal, reference numeral 32 represents a interference-cancelled reception signal transmission line for transferring an interference-cancelled reception signal to an adjacent stage, reference numeral 33 represents an interference canceller unit ICU, reference numeral 34 represents a delay memory, reference numeral 35 represents a subtracter for subtracting interference replica components from the reception signal, reference numeral 36 represents an interference replica signal transferring line for transferring the interference replica signal of each user to its own user (present user) of a succeeding stage, reference numeral 37 represents an adder for adding a (interference) replica signal of the present user of a previous stage again (because the signal components of all the users of the first stage are subtracted), reference numeral 38 represents a delay (memory) for holding the (interference) replica signal of the present user of the previous stage, and reference numeral 39 represents a decoder for performing the final decoding.
Next, the operation of the multi-user serial interference canceller shown in FIG. 4 will be described.
A reception signal r31 is first input to ICU#11 (Interference Cancellation Unit) 33 of a first user in a first stage, and an interference replica signal S11 on the line 36 thereof is generated in ICU#11. Subsequently, in a subtracter 35, the interference replica signal S11 generated from the reception signal r31 is subtracted from the signal r31 which is delayed by the delay 34 for delaying the signal by the processing time in ICU#11, and the subtraction result is input to ICU#12 of a second user in the first stage. A signal [r31xe2x88x92S11] is input to ICU#12, and an interference replica signal S12 on the line 36 is output. Subsequently, the signal of a k-th user in an i-th stage which is represented by the following equation is generally transmitted to an adder 37 which is connected to ICU#k of the k-th user in the i-th stage:
yk,1(m)=∫mT+xcfx84k,1(m+1)T+xcfx84k,1Ck(txe2x88x92xcfx84k,1)Ck*(txe2x88x92xcfx84k,1)xe2x80x83xe2x80x83(1)
As is apparent from the equation (1), the equation (1) contains the reception signal r31 at all times, and thus it is unnecessary to transfer the reception signal by another system and holds the reception signal r31. Further, the signal in ICU#ik is a residual signal obtained by subtracting the components of all the users from the reception signal r31. Therefore, the signal S(ixe2x88x921)k, that is, the interference replica signal of the present user obtained at the previous stage is added again and then input to ICU before the processing for the k-th user in the i-th stage is carried out. AR of these signals are chip rate signals.
FIG. 5 shows the internal construction of ICU#ik based on the conventional system.
A reception signal r(t) input is despreaded by a spread code Ck(t) of the present path of the present user, and then integrated in an integrator 42 to examine a correlation therebetween. A transmission path fading vector "xgr" is calculated from the integrated signal in a transmission estimator 43, and then the inverse "xgr"xe2x80x2 of the transmission path fading vector is multiplied by the integrated signal in a multiplier 54 to perform phase correction. The signal which is subjected to the phase correction every path is subjected to RAKE combining in a RAKE combiner 45. The signal output from the RAKE combiner 45 is output to a decoder and also is used to decode an original symbol sequence in a data-decision unit 46. Thereafter, the original symbol sequence is multiplied by the transmission path fading vector "xgr" every path in a multiplier 47 to be returned to an original transmission characteristic. The original transmission characteristic (Ck(t)) is multiplied/spread in a multiplier 48 to reproduce the interference replica signal of the chip rate, and then the interference replica signal is transferred to a succeeding user or a succeeding stage.
As shown in FIG. 4, in the final stage, the interference-cancelled signal is input to a decoder 39, and the final decoding result is output.
Japanese Laid-open Patent Application No. Hei-9-270766 discloses a transmission system for interference replica signals of DS-CDMA multi-user serial interference canceller as in the case of the above publication. According to this publication, the interference replica signal of each user is transmitted through a common bus line to be publicly inform to all the other users than the present user, and thus the signal having a transmission speed of (Kxc3x97N) times of the transmission speed (=n-sampling times of spreading rate) of the interference replica signal of one user would flow on the common bus line if no modification is made. For example, if the transmission speed of the interference replica signal is equal to 120 Mbps, the number of users is equal to 16 and the number of stages is equal to 3, the transmission speed needed for the common bus line is equal to 5.76 Gbps, and this means that it is difficult to implement a circuit which satisfies this condition. Accordingly, this publication discloses a technique in which the signal is transmitted between adjacent users through a directly-connected parallel bus line or the like without using a common bus line, the processing is successively performed, the sum of the processing results is transferred, and the interference signal components of the present user is transferred to a different transmission line and then subtracted, thereby reducing the transmission speed. However, it is very difficult to achieve simplification in construction by using the technique of this publication because it uses the parallel bus line.
In general, the DS-CDMA communication system, the chip rate, that is, the frequency of spread spectrum is high, and the multiplexing number of users can be increased as the frequency is spread to a broader band, thereby enabling high-speed data communications. Therefore, a communication system having a high chip rate has been recently required to be developed.
In the conventional type multi-user interference canceller, generation of interference replica signal, subtraction of the reception signal thereof, etc. are performed every spectrum-spread chip rate. That is, the reception signal is temporarily subjected to despreading to perform an transmission path estimation therefor and then reproduce interference components, and then the spreading is performed again because the interference components thus reproduced are subtracted from the reception signal (which is the chip rate and a multiplexed signal of all the user components).
Therefore, as the chip rate is higher, the interference canceller is required to perform the interference replica signal generation processing at a higher speed, and also the signal transmission amount between user cards is increased more and more.
Further, since the conventional system performs the processing on a chip-rate basis, band restriction is imposed on the reception signal to be processed by a transmission/reception band filter. Therefore, it is also necessary to impose the same band restriction on the interference replica signal thus generated, and thus a high-speed band restriction filter is needed, so that the device itself must be designed in a large scale.
Still further, a delay memory for holding the interference replica signal of the present user while it is left as the chip rate, and a delay memory for holding an interference replica signal of one stage of the present user of the previous stage are needed. In this case, the data to be held is the information on the chip rate, and thus a large capacity of memory is generally needed, and thus the enhancement in chip rate speed causes the increase of the capacity of the memory.
An object of the present invention is to simplify the construction of a DS-CDMA multi-user interference canceller, and provide the construction which can cancel the interference of the other users. Further, another object of the present invention is to avoid complication in device construction and increase in device scale due to interference generation and interference cancel at conventional chip rate with respect to the constructing method of a DS-CDMA multi-user interference canceller.
In order to attain the above objects, the present invention is characterized in that the signal of each user is first subjected to despreading in a correlator to be converted to a symbol rate signal, and then all of the processing of generating interference replica signal, the processing of subtracting the interference replica signal from the reception signal, etc. are performed on a symbol rate basis.
Specifically, according to an aspect of the present invention, a DS-CDMA (Direct Sequencexe2x80x94Code Division Multiple Access) multi-user interference canceller, is characterized by comprising: notifying means for notifying symbol information, a path-based delay and path-based transmission path estimation value information to other users; correlation value calculating means for beforehand calculating cross correlation values with spread code sequences of the other users in each user; interference calculating means for calculating interference components of the other users on the basis of the information notified from the other users and the cross correlation values which are calculated in advance; and means of subtracting the interference components from the signal component of the present user, thereby performing the interference component generation and cancellation at a symbol rate after the despreading of the reception signal is carried out.
Further, according to another aspect of the present invention, a DS-CDMA multi-user interference canceller for receiving spread signals from plural users and canceling the interference components between the respective data of the spread signals thus received to decode the data transmitted, is characterized by comprising: each correlator for receiving the spread signals from many users and subjecting each spread signal to despreading with a spread code sequence for each user to obtain a correlation value; interference cancellers of plural stages for subtracting the other interference replica from the correlation value of the correlator every user and performing RAKE combining; and a decode of each user for decoding the data from the RAKE-composed signal of the last stage of said interference canceller.
Describing the present invention in more detail, FIG. 1 shows the overall construction of the present invention, and FIG. 2 shows the internal construction of ICU having the feature of the present invention. The present invention is characterized in that IRU (Interference Regeneration Unit) for generating an interference component imposed on the other users by the reception data of a user which is converted to a symbol rate in a correlator, and a subtracter for subtracting the interference component from the reception data of the symbol rate are provided, and the interference components of the other users are removed by directly using the symbol data.
Further, the correlator is mounted in only the first stage, and the output thereof is input (every path) to ICU of each stage.
The interference regeneration unit IRU executes the operation of beforehand calculating the cross correlation of the spread codes between the respective users on the basis of the spread code sequence of each user and the delay of each path which are held in advance, thereby calculating the interference component of each path of each user.
Accordingly, according to this system, the re-spreading when the interference replica is generated is not required, and the signal transmission between cards may be performed at a relatively low speed. Further, the band limiting filter after the re-spreading is not required, and thus the enlargement of the device scale can be prevented.